1. Field of the Invention
This invention relates to an apparatus for heating samples of biological material, and more particularly an apparatus for thermal cycling of DNA samples to accomplish a polymerase chain reaction, a quantitative polymerase chain reaction, a reverse transcription-polymerase chain reaction, or other nucleic acid amplification types of experiments.
2. Description of the Related Art
Currently, techniques for thermal cycling of DNA samples are well-known. By performing a polymerase chain reaction (PCR), DNA can be amplified. It is desirable to cycle a specially constituted liquid biological reaction mixture through a specific duration and range of temperatures in order to successfully amplify the DNA in the liquid reaction mixture. Thermocycling is the process of melting DNA, annealing short primers to the resulting single strands, and extending those primers to make new copies of double stranded DNA. The liquid reaction mixture is repeatedly put through this process of melting at high temperatures and annealing and extending at lower temperatures.
In a typical thermocycling apparatus, a biological reaction mixture including DNA will be provided in a large number of sample wells on a thermal block assembly. It is desirable that the samples of DNA have temperatures throughout the thermocycling process that are as uniform as reasonably possible. Even small variations in the temperature between one sample well and another sample well can cause a failure or undesirable outcome of the experiment. For instance, in quantitative PCR, one objective is to perform PCR amplification as precisely as possible by increasing the amount of DNA that generally doubles on every cycle; otherwise there can be an undesirable degree of disparity between the amount of resultant mixtures in the sample wells. If sufficiently uniform temperatures are not obtained by the sample wells, the desired doubling at each cycle may not occur. Although the theoretical doubling of DNA rarely occurs in practice, it is desired that the amplification occurs as efficiently as possible.
In addition, temperature errors can cause the reactions to improperly occur. For example, if the samples are not controlled to have the proper annealing temperatures, certain forms of DNA may not extend properly. This can result in the primers in the mixture annealing to the wrong DNA or not annealing at all. Moreover, by ensuring that all samples are uniformly heated, the dwell times at any temperature can be shortened, thereby speeding up the total PCR cycle time. By shortening this dwell time at certain temperatures, the lifetime and amplification efficiency of the enzyme are increased. Therefore, undesirable temperature errors and variations between the sample well temperatures should be decreased.
In light of the foregoing, there is a need for a thermocycling apparatus that enhances temperature uniformity for the DNA sample wells in the apparatus.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be appreciated by practice of the invention. The advantages and purposes of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, the invention includes an apparatus for heating samples of biological material. The apparatus in its preferred embodiment includes: a thermal block assembly including a plurality of sample holders for receiving samples of biological material; a heat sink thermally coupled to the thermal block assembly, the heat sink transferring heat away from the thermal block assembly to ambient air in contact with the heat sink; a first heat source thermally coupled to the thermal block assembly to provide heat to the thermal block assembly; and a second heat source thermally coupled to the first heat source and configured to provide heat to at least a portion of the first heat source. The arrangement of the heat sink, first heat source and second heat source can provide substantial temperature uniformity among the plurality of sample holders.
In another aspect, the apparatus includes: a thermal block assembly including a plurality of sample wells for receiving samples of biological material; and a first cover of insulating material. The first cover tends to thermally insulate the sample wells of the thermal block assembly. The first cover includes a plate with a plurality of cylindrical sample well openings. Each cylindrical sample well opening corresponds to a respective sample well. The first cover surrounds the top and extends over at least a portion of the sides of the thermal block assembly.
In a further aspect of the invention, the invention includes a method for thermally cycling samples of biological material in an apparatus with at least one sample holder located in a thermal block assembly. The method includes the steps of inserting at least one sample of biological material into a sample holder of the apparatus; measuring the temperature of the thermal block assembly at at least one location on the thermal block assembly; calculating the desired temperature of the thermal block assembly; comparing the desired temperature with the measured temperature, and if the measured temperature is less than the desired temperature, the method further comprises the steps of: applying a first heat source, a portion of the heat from the first heat source being transferred to the thermal block assembly; applying a second heat source, a portion of the heat from the second heat source being transferred to the first heat source; and applying a third heat source, a portion of the heat from the third heat source being transferred to the sample holders; if the measured temperature is greater than the desired temperature, the method further comprises the step of cooling the thermal block assembly by imparting a cooling convection current on a heat sink which is thermally coupled to the thermal block assembly to provide heat transfer from the thermal block assembly to ambient air in contact with the heat sink; and repeating the steps of measuring, calculating, and comparing until the predetermined thermal cycle for the samples of biological material is completed.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.